1. Field of the Invention
This invention generally relates to an LED color illumination apparatus for a polarized light projection system and more specifically to a light projection system for providing uniform light distribution to a display panel.
2. Background of the Invention
In recent years, digital projection systems using spatial light valve modulators, such as a digital micromirror device (hereafter “DMD”), transmissive liquid crystal display (hereafter “LCD”) and reflective liquid crystal on silicon (hereafter “LCoS”) have been receiving much attention as they provide a high standard of display performance. These displays offer such advantages as high resolution, a wide color gamut, high brightness and a high contrast ratio.
Color projection systems of the type based on either LCD technology or LCoS technology require linearly polarized light as the illumination light source. LCD and LCoS devices depend on either the polarization rotation effect or the birefringent effect of the liquid crystal to generate light. The light emitted from the light source must be converted into polarized light for illuminating an LCD or LCoS spatial light modulator. Those skilled in the art will recognize that the optical system contained within a commercial LCD or LCoS projector typically combines a fly's-eye lens array with a polarizing beam splitter array. Examples of such an arrangement can be found in U.S. Pat. Nos. 6,411,438, 6,776,489, 6,739,726 and 6,092,901 which are all incorporated by reference herein. Two drawbacks to using the fly's-eye type of optical system are that it is bulky and expensive to manufacture.
Most projection systems use short arc gaseous white lamps such as ultra-high pressure mercury, xenon or the like that can achieve a relatively high etendue efficiency required for panel illumination. Etendue refers specifically to the geometric capability of an optical system to transmit radiation such as its throughput. Currently, only a limited number of manufacturers are capable of producing high-quality short arc lamps. The typical operational lifetime of these types of lamps is about 2000 to 6000 hours. Moreover, there are significant amounts of ultraviolet (UV) and infrared (IR) light emitted from this type of lamp. The unfiltered UV light reduces the lifetime of both the optical components and microdisplay panel within the system, while IR light requires additional cooling devices to maintain a desired operating temperature.
Significant efforts have been dedicated towards moving away from short arc lamps through the utilization of light emitting diodes (LED) in projection illumination systems. One apparent advantage is that LEDs using three primary colors can produce a wider color gamut than conventional white lamps. In addition, LEDs have a high light efficiency, i.e., the ratio of luminous output to the electrical power required, since all spectra of the red, green and blue light from LEDs can be utilized in a visual system. U.S. Pat. No. 6,224,216, which is incorporated by reference herein, describes a triple-path projector employing three single color LED arrays. The LEDs emit light propagating along separate paths through fiber bundles to respective waveguide integrators and thereafter to respective display devices. A problem exists in this type of system because of the coupling between LEDs and fibers. In practice, due to coupling and transmission loss, it is difficult to efficiently couple light emitting from the LED arrays to the corresponding fiber bundles and waveguides.
Similarly, U.S. Pat. No. 6,220,714 discloses a projection system using LEDs for illumination, where light beams emitting from red, green and blue LED arrays are collimated by condenser lenses which pass through fly's-eye type integrators for illuminating a single panel. Based on the geometry of the fly's-eye type integrator, only the surface area of light emitting region within a certain field of view can be effectively collected for illuminating a panel. A similar system can be found in U.S. Pat. No. 6,644,814, which describes an LED-illumination-type DMD projector with one panel. Generally, a common problem with these prior art systems is that some light from LEDs cannot enter the corresponding lens of the first and second fly's-eye lenses due to aberration and aperture limitation of the lens array. Therefore, a portion of the illumination light will fall outside the panel area, resulting in low light efficiency and low contrast.
Research has also been conducted on using light pipes as means of collecting and homogenizing light for polarize illumination applications. For example, U.S. Pat. No. 6,587,269 discloses a waveguide polarization recovery system comprising an input waveguide that inputs non-polarized light energy into the system. A polarizing beam splitter receives light energy from an input waveguide and transmits light energy of a first polarization type and reflects light energy of a second polarization type. A wave plate modifies the polarization of the transmitted or reflected light energy and an output waveguide removes polarized light energy from the previous system. This type of waveguide polarization recovery system was designed for use with white light sources so that multi-color light sources and beam combiner are not required.
Thus, there is a need to provide a light illumination device for LCD or LCoS projection systems or the like which utilizes polarized light with high efficiency and adequate brightness without utilizing complicated and/or expensive components.